Storage system and method for acquisition and utilisation of snapshots

ABSTRACT

There are provided a mainframe  1  constituting a host device and a disk array system  2  having logical volumes (LDEV)  30  connected with the mainframe  1 . The mainframe  1  is provided with an input/output device configuration definition information storage section  14  that stores the address of an LDEV  30   a . By referring to the input/output device configuration definition information storage section  14 , a command is then issued giving an instruction to perform back-up of the LDEV  30   a  specifying the address of the LDEV  30   a . The disk array system  2  receives the back-up command, specifies the generation to which the LDEV  30   a  is to be backed up and backs up to an LDEV  30   b  to  30   d  corresponding to the specified generation.

CROSS-REFERENCE TO PRIOR APPLICATION

This application relates to and claims priority from Japanese PatentApplication No.2004-168262, filed on Jun. 7, 2004, the entire disklosureof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

In a system wherein a mainframe accesses input/output devices inaccordance with configuration definitions of the input/output devicesthat are registered beforehand, the present invention relates to atechnique for taking a plurality of generations of snapshots in aflexible manner without altering the configuration definitions of theinput/output devices.

In general, when a mainframe performs input/output of data in respect ofinput/output devices such as disk volumes or tape volumes, it performsthis input/output of data in accordance with the configuration of theinput/output device that is registered in the operating system (OS). Thesystem administrator must therefore perform beforehand the task referredto as configuration definition of the input/output devices. Typically,configuration definition of the input/output devices involves definitionof the bus whereby the device performs input/output, definition of thenumber of the control device of the input/output device and definitionof the storage device address. This makes it possible for theseinput/output devices to be used from the mainframe by putting theseinput/output devices into a condition in which they are logicallyincorporated with the mainframe to constitute the system.

Also, an upper limit on the number of input/output devices i.e. thenumber of device addresses that can be defined is laid down, dependingon the type of mainframe and OS. Mainframes therefore cannot use anumber of input/output devices that exceeds this upper limiting value.

Also, the amount of data that needs to be saved continues to increasewith increase in the amounts of data that are accumulated. In manycases, the practice is adopted that data that are constantly accessed bya client business are saved on a disk array system and after thebusiness that makes use of these data has finished with them, a back-upof the data is saved on a tape system. With this practice, if thereshould be any corruption of the data stored on the disk array system duefor example to a mistake in operation, the data can be restored to theoriginal condition by using the back-up acquired by the tape system.However, the time required for acquiring this back-up is proportional tothe amount of data.

Furthermore, in recent years, with increases in the amount of data andincreased penetration of 24 hour continuous service, cases have startedto arise in which sufficient back-up time to back-up the data of thedisk array system to a tape system cannot be secured. Cases in which useis made of so-called “snapshots” as a technique for maintaining aback-up of the data in question at a given time-point in a disk arraysystem have therefore become more frequent. A “snapshot” as referred tohere indicates a logical copy for restoring the data condition at agiven time-point. If the practice is adopted of taking snapshots at ashigh a frequency as possible i.e. with time intervals that are short aspossible, if it should subsequently be necessary to restore the data,the restoration time can be shortened by employing a snapshot that iscloser to the time-point to which restoration is desired.

SUMMARY OF THE INVENTION

However, in the case of a mainframe, even though a snapshot is acquired,unless the snapshot volume in question is defined from the mainframe asan input/output device, the snapshot in question cannot be used. It istherefore necessary to define the snapshot volume beforehand from themainframe. For example, if one week's worth of snapshots, taken everytwo hours, of a single volume of data are held, it is necessary that forexample 84 snapshot volume addresses should be registered asconfiguration definitions.

In general, alteration of the configuration definitions is not performedfrequently, being only carried out in restricted circumstances such aswhen volumes are added or subtracted by adding or subtracting equipment;it is desirable that the number of times that alteration of theconfiguration definitions of the system is carried out is small, sincethis is something that requires the time of the system administrator.

Accordingly, an object of the present invention is to provide atechnique for managing a plurality of generations of snapshots ofinput/output devices on a mainframe in a flexible manner withoutaltering the configuration definitions of the input/output devices.

A storage system according to an embodiment of the present inventionconsists in a storage system that is accessed from a host device,comprising: a storage device comprising a plurality of logical volumes;a storage device control section that controls said storage device; aninterface section that receives a snapshot acquisition instruction andsnapshot utilization instruction from said host device; and a snapshotprocessing device that executes processing for acquisition andutilization of snapshots in accordance with said snapshot acquisitioninstruction and said snapshot utilization instruction received by saidinterface section. The snapshot processing device, in response to asnapshot acquisition instruction, selects a logical volume that iscapable of use as a snapshot target from the plurality of logicalvolumes; instructs the storage device control section to acquire asnapshot of the logical volume of the snapshot source, designated in thesnapshot acquisition instruction for the selected logical volume, andholds generation information that associates the generation numberspecified in the snapshot acquisition instruction with the address ofthe selected logical volume. In addition, in response to a snapshotutilization instruction, the snapshot processing device specifies alogical volume where the snapshot corresponding to the generation numberspecified in the snapshot utilization instruction is held, by referringto the generation information, and allocates an address accessible bythe host device to the specified logical volume.

In a preferred embodiment, the address accessible by the host devicethat is allocated to the specified logical volume is a logical volumeaddress that is registered beforehand in the host device.

In a preferred embodiment, when the address accessible by the hostdevice is allocated to the specified logical volume, the snapshotprocessing device adds information indicating the fact that the addressaccessible by the host device has been allocated to the specifiedlogical volume to the generation information.

In a preferred embodiment, the snapshot acquisition instruction mayfurther include designation of the logical volume of the snapshottarget. The snapshot processing device may then, in response to thesnapshot acquisition instruction, if the snapshot target logical volumedesignated in the snapshot acquisition instruction is already used,select an unused logical volume from the plurality of logical volumesand give an instruction to the storage device control section to acquirea snapshot of the snapshot source logical volume designated in thesnapshot acquisition instruction for the selected logical volume.

In a preferred embodiment, the snapshot utilization instruction mayfurther include designation of the logical volume of the snapshottarget. The snapshot processing device may then, in response to thesnapshot utilization instruction, by referring to the generationinformation, if a snapshot of the generation number specified in thesnapshot utilization instruction has not been acquired by the snapshottarget logical volume specified in the snapshot utilization instruction,allocate the logical volume address of the specified snapshot target tothe logical volume where the snapshot of the designated generationnumber is held. In addition, it may be arranged to give notification tothe host device of the fact that a snapshot of the specified generationnumber has become available for use.

A storage system according to an embodiment of the present inventionconsists in a storage system that is accessed from a host device,comprising: a storage device comprising a plurality of logical volumes;a storage device control section that controls said storage device; aninterface section that receives a snapshot acquisition instruction fromsaid host device, a snapshot utilization instruction and a generationinformation acquisition instruction; and a snapshot processing devicecomprising a generation information storage section that storesgeneration information indicating the snapshot acquisition status foreach generation and that executes processing in accordance with saidsnapshot acquisition instruction, said snapshot utilization instructionand generation information acquisition instruction received by saidinterface section. The snapshot processing device, in response to thesnapshot acquisition instruction, selects one logical volume from theplurality of logical volumes; it then gives an instruction to acquire asnapshot of the logical volume of the snapshot source designated in thesnapshot acquisition instruction for the selected logical volume. Inaddition, the snapshot processing device, by referring to the generationinformation storage section, specifies a generation number andassociates this specified generation number with the address of theselected logical volume and stores these in the generation informationstorage section. The snapshot processing device also acquires theinformation stored in the generation information storage section inresponse to the generation information acquisition instruction and givesan instruction to the interface section to send the information to thehost device. The snapshot processing device also, in response to thesnapshot utilization instruction, by referring to the generationinformation storage section, specifies the logical volume where asnapshot has been acquired corresponding to the generation numberspecified in the snapshot utilization instruction. An address accessibleby the host device is then allocated to this specified logical volume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a computer system according to an embodiment ofthe present invention;

FIG. 2 is a view showing an example of input/output device configurationdefinition information;

FIG. 3 is a functional layout diagram of a disk array system;

FIG. 4 is a view showing an example of generation managementinformation;

FIG. 5 is a flow chart showing the sequence of snapshot acquisitionprocessing;

FIG. 6 is a view showing an example of when a snapshot command is inputfrom the command line;

FIG. 7 is a flow chart showing the processing sequence for making asnapshot utilizable;

FIG. 8 is a flow chart showing the sequence of snapshot acquisitionprocessing;

FIG. 9 is a flow chart of the processing sequence for making a snapshotutilizable; and

FIG. 10 is a view showing an example of a screen for selecting asnapshot generation.

DETAILED DESCRIPTION OF THE INVENTION

A computer system according to a first embodiment of the presentinvention is described below with reference to the drawings.

First of all, FIG. 1 is a view illustrating diagrammatically the entirelayout of this system. This system comprises a mainframe 1 constitutinga host device and a disk array system constituting an external storagedevice of the mainframe 1.

The disk array system 2 comprises a disk device 21 comprising aplurality of physical volumes (hereinbelow called PDEV) 21 a and acontrol device 22 that performs control of the disk device 21. In thedisk device 21, a plurality of logical volumes (hereinbelow calledLDEVs) 30 (30 a to 30 d) are constituted spanning the plurality of PDEVs21 a. The control device 22 manages the disk device 21 for each LDEV 30,respective logical addresses being allocated to the LDEVs 30.

An operating system (OS) 11, a snapshot management program 12, and aprescribed application program 13 are installed on the mainframe 1. Thesnapshot management program 12 performs processing such as taking asnapshot of a prescribed LDEV of the disk array system 2 periodically orwith any desired timing and generating a command for making a snapshotutilizable.

Also, input/output device configuration definition information(hereinbelow sometimes simply referred to as configuration definitioninformation) 14 is registered beforehand in the OS 11. FIG. 2 shows anexample of the input/output device configuration definition information14. Specifically, the configuration definition information 14 comprisesas data items a bus ID 141 of the bus that is set as the access path inrespect of the disk array system 2, the number 142 of the control device22 in the disk array system 2 that is accessed by each bus, and theaddress 143 of an LDEV 30 that is accessible by each bus.

The mainframe 1 can access the disk array system 2 by designating theaddress of the LDEV that is to be accessed, by referring to theinput/output device configuration definition information 14. Themainframe 1 can therefore only access an LDEV that is previously definedin the input/output device configuration definition information 14. Inthe example of FIG. 2, the structure in the range enclosed by the brokenline of FIG. 1 is defined. In this case, the mainframe can thereforeonly access the LDEVs 30 a and 30 b and cannot directly access the otherLDEVs 30 c, 30 d, . . . .

Next, the functional layout of the disk array system 2 will be describedin detail with reference to the block diagram shown in FIG. 3. As shownin this Figure, the control device 22 of the disk array system 2comprises an interface control section 23, shared memory 24, diskcontrol section 26 and a cache memory 27 that are mutually connected bymeans of a bus 28. The interface control section 23 comprises aprogrammed microprocessor 29; a mainframe communication section 29A thatperforms input/output of information with respect to the mainframe, asnapshot processing section 29B and a bus input/output control section29C that controls data input/output with respect to the bus 28 areimplemented by controlling the microprocessor 29. The disk controlsection 26 comprises a programmed microprocessor 26A and performscontrol of the disk device 21. The shared memory 24 is constituted byfor example non-volatile memory and stores generation managementinformation 25. The shared memory 24 and the cache memory 27 may beconstituted by respectively different memories or may be constituted onthe same memory. The bus 28 may be constituted by a high-speed bus suchas a very high-speed cross bus, in which data transfer is performed by ahigh-speed switching action.

When for example the disk array system 2 issues a command for themainframe 1 to execute data input/output in respect of the LDEV 30 a(DEVA #00), this is received by the mainframe communication section 29A.If for example the received command is an instruction to input (write)data in respect of the LDEV 30 a, the bus input/output control section29C stores the data to be written in the cache memory 27 and the diskcontrol section 26 writes the data in the LDEV 30 a asynchronously. Onthe other hand, if the received command is an instruction to output(read) data in respect of the LDEV 30 a, the disk control section 26, inaccordance with an instruction from the bus input/output control section29C, reads the designated data from the LDEV 30 a and stores it in thecache memory 27 and the bus input/output control section 29C acquiresthis data from the cache memory 27 and outputs it to the mainframe 1.

Also, the disk array system 2 according to this embodiment performsgeneration management of a plurality of snapshots. For example, when themainframe communication section 29A receives a snapshot acquisitioncommand and a utilization command from the mainframe 1, the snapshotprocessing section 29B executes various types of processing necessaryfor acquisition and utilization of a snapshot, as will be described.

In fact, when a snapshot is acquired, the disk control section 26acquires a snapshot when the snapshot processing section 29B gives aninstruction for performance of back-up (i.e. snapshot acquisition),designating the address of the main volume (snapshot source volume) thatis the subject of back-up and the address of an auxiliary volume(snapshot target volume) that is the back-up target.

The generation management information 25 indicates the acquisitionstatus of each generation of snapshots of the LDEV that is the subjectof back-up. FIG. 4 shows an example of the information that is stored inthe generation management information 25. Specifically, the data itemsof the generation management information 25 comprise the address 251 ofthe main volume, the address 252 of the auxiliary volume, the generationnumber 253 that identifies the generation, the LDEV addresses 254 foreach generation allocated for a snapshot of the various generations, theutilization flag 255 and the snapshot acquisition time point 256.

The address 251 of the main volume is the address of the LDEV that isthe subject of back-up. In this case, the main volume 251 is the LDEV 30a, so its address DEVA #00 is stored.

The address 252 of the auxiliary volume indicates the address of theLDEV that is recognized an auxiliary volume by the mainframe 1. That is,although, in this embodiment, a plurality of generations of snapshotscan be managed, as will be described, the only back-up target auxiliaryvolume that can be recognized by the mainframe 1 is the LDEV 30 b. Inthis case, DEVA #01 is therefore stored in the address 252 of theauxiliary volume.

The generation number 253 is the generation number that is assigned toeach snapshot.

The LDEV addresses 254 for each generation are the addresses of LDEV 30that are allocated to each respective generations. The LDEV addresses254 for each generation are allocated beforehand to each generation;they may be registered in the generation management information 25 orthe necessary generation address may be allocated when the snapshot isacquired.

The utilization flag 255 is a flag for identifying generations that arecapable of being utilized from the mainframe 1. The condition of beingutilizable from the mainframe 1 i.e. that the address 252 of theauxiliary volume is temporarily allocated is a condition in which datainput/output from the mainframe 1 is feasible. In the example of FIG. 4,for example the snapshot of generation number 3 is utilizable.

In a system comprising a construction as described above, the processingfor acquiring snapshots of the LDEV 30 a for each generation will bedescribed with reference to the flow chart of FIG. 5.

First of all, in the main frame 1, in response to an instruction from anapplication 13.or an instruction from the user using an input device 10a, the snapshot management program 12 issues (S11) a snapshotacquisition command in respect of the disk array system 2. The snapshotacquisition command may include for example an address and generationnumber for specifying the main volume and auxiliary volume. For example,FIG. 6 shows an example of the case where a snapshot acquisition commandis input by the user on the command line 200. In this case, on thecommand line 200, following the snapshot acquisition command, the mainvolume address 210, the auxiliary volume address 220 and the generationnumber 230 are input. Thus the information that is input on the commandline 200 is included in the snapshot acquisition command.

The control device 22 of the disk array system 2 receives this snapshotacquisition command. The snapshot processing section 29B then, byreferring to the generation management information 25, determineswhether the address of the auxiliary volume that is contained in thesnapshot acquisition command is registered in the LDEV addresses 254 foreach generation i.e. determines whether or not this address is alreadyused (S12). If the address is already used (S12: Yes), the snapshotprocessing section selects one of the unused LDEVs (S13).

If the address is unused (S12: No), and if an unused LDEV is selected inthe step S13, the address of this LDEV is stored in the LDEV addresses254 for each generation, associated with the generation number containedin the snapshot acquisition command, and the generation managementinformation is updated (S14). The snapshot processing section 29B thengives an instruction for execution of back-up to the disk controlsection 26, by designating the addresses of the main volume andauxiliary volume. The disk control section 26 thereby acquires asnapshot f (S15). When a snapshot has been acquired, the snapshotprocessing section 29B notifies the mainframe 1 of completion ofacquisition of a snapshot (S16).

It is thereby possible to acquire a snapshot to a volume that themainframe 1 does not directly recognize. As a result, anumber ofsnapshots equal to or greater than the number of volumes registered inthe configuration definition information of the mainframe 1 can beacquired for each generation.

Next, processing for utilization of one of the snapshots acquired by theabove processing will be described with reference to the flow chart ofFIG. 7.

First of all, the snapshot management program 12 issues (S21) a snapshotutilization command in accordance with information that is input by theuser from an input device 10 a or in accordance with informationreceived as an instruction from an application 13. The snapshotutilization command includes for example the address of the LDEV (inthis case, DEVA #00) that is the subject of back-up, the snapshotaddress (in this case, DEVA #01) and the snapshot generation number thatis desired to be utilized. This snapshot utilization command is acommand to make it possible for the mainframe 1 to utilize the snapshotof the designated generation of the LDEV 30 a that is the subject ofback-up as DEVA #01.

The control device 22 of the disk array system 2 receives this snapshotutilization command. The snapshot processing section 29B then determines(S22) whether the designated generation number is associated with thesnapshot address designated in the snapshot utilization command, byreferring to the generation number 253 of the generation managementinformation 25 and the LDEV addresses 254 for each generation.

If the designated generation number is not associated with thedesignated snapshot address (S22: No), the mainframe 1 cannot directlyaccess the snapshot volume of the designated generation. For example, if“3” is designated as the generation number by the snapshot utilizationcommand, the address of the generation number “3” found in the LDEVaddresses 254 for each generation is “DEVA#03”, so the main frame 1cannot access the snapshot volume of generation number “3”. The snapshotprocessing section 29B therefore specifies (S23) an item in the LDEVaddresses 254 for each generation (in the above example, DEVA #03)corresponding to the designated generation number, by referring to thegeneration management information 25.

The address (DEVA#01) designatedby the snapshot utilization command isthen allocated (S24) to the snapshot volume of this specified address.That is, in the above example, DEVA #03 is converted to DEVA #01.

The snapshot processing section 29B then notifies (S26) the mainframe 1that the snapshot volume of the designated generation has becomeutilizable by setting the utilization flag 255 of the generation number(3 in the above example) specified by the generation managementinformation 25 to ON (S25).

In this way, the mainframe 1 can access the snapshot volume of thedesignated generation as the already-defined DEVA #01 in theconfiguration definition 14. As a result, it becomes possible to utilizesnapshot volumes from a large number of snapshot volumes of volumenumber greater than that defined in the configuration definition 14 byspecifying their generation number.

Next, a second embodiment of the present invention will be described. Inthis embodiment, the system layout shown in FIG. 1 to FIG. 4 is the sameas that of the first embodiment. However, in this embodiment, theprocessing sequence for acquisition and utilization of a snapshot isdifferent from that of the first embodiment. This processing sequence isdescribed below with reference to FIG. 8 to FIG. 10.

FIG. 8 is a flow chart showing the snapshot acquisition processingsequence according to this embodiment. First of all, the snapshotmanagement program 12 of the main frame 1 issues (S31) a snapshotacquisition command in respect of the disk array system 2, in responseto an instruction from the application 13 or an instruction from theuser using the input device 10 a, in the same way as in the firstembodiment. This snapshot acquisition command includes at least theaddress of the main volume and may further include the address of theauxiliary volume.

When the control device 22 receives this back-up command, the snapshotprocessing section 29B, by referring to the generation managementinformation 25, specifies (S22) the generation number of the generationat which a back-up is to be taken. For example, in this embodiment, theLDEV addresses 254 for each generation are allocated beforehand to eachgeneration number 253. Then, if there is a generation which is unusedi.e. whose back-up time-point 256 is blank, this generation is specifiedas the generation to be used in this case or, if all the generations areused, the generation to be used is specified as the generation whoseback-up time point 255 is earliest.

When the generation number at which a back-up is to be taken isspecified in step S32, the snapshot processing section 29B, by furtherreferring to the generation information storage section 25, sets (S33)the item in the LDEV addresses 254 for each generation that isassociated with this generation number 253 as the address of theauxiliary volume. At this point, if the address of the auxiliary volumeis specified in the snapshot acquisition command, the specified addressis diskarded and the aforesaid address is substituted.

The snapshot processing section 29B then acquires a snapshot (S44) byperforming back-up to the auxiliary volume that was set in step S13 fromthe main volume that was designated by the snapshot acquisition command.In this way, the disk device 21 acquires a snapshot of the main volume,constituting a snapshot of the designated generation.

The snapshot processing section 29B then updates (S45) the generationmanagement information 25 in accordance with the snapshot acquisitionprocessing described above. Specifically, the snapshot processingsection 29B updates the snapshot acquisition time point 256corresponding to the generation number 253 of the acquired snapshot.

When the above processing has been completed, the disk control section26 notifies the mainframe 1 of the fact that back-up has been completed(S46).

In this embodiment, a snapshot is acquired by automatically performinggeneration allocation when the disk array system 2 receives a snapshotacquisition command. That is, the user does not need to be conscious ofthe generation, since it is not necessary to specify to which generationback-up should be performed at the mainframe 1.

Next, the processing for utilization of one of the snapshots acquired bythe above processing will be described with reference to the flow chartof FIG. 9.

First of all, the snapshot management program 12 issues (S41) a requestfor acquisition of generation management information held by the diskarray system 2 in respect of a main volume designated by the user usingthe input device 10 a, or an LDEV that has been designated from anapplication 13. The designation of the LDEV is performed by for exampledesignating its address.

When the control device 22 receives this generation informationacquisition request, the snapshot processing section 29B acquires thegeneration information of the LDEV designated from the generationmanagement information 25 and sends this to the mainframe 1 (S42).

The snapshot management program 12 displays (S43) on a display device 10b a screen for the user to select a generation in accordance with thegeneration management information sent from the disk array system 2.FIG. 10 shows an example of this selection screen 100. The generationselection screen 100 includes, as shown for example in this Figure, atable 110 of snapshot acquisition time points for each generation and aregion 120 that accepts input of a generation number to be utilized.Although, in this case, only the snapshot acquisition time point isdisplayed for each generation number of the generation managementinformation, it would be possible for the screen to also displaygeneration management information other than this.

When the user selects a generation number on this generation selectionscreen, a snapshot utilization command to make it possible to utilize asnapshot of the designated generation number is issued (S44) in respectof the disk array system 2. In the case of the screen 100 of FIG. 8,when the user inputs the generation number of the snapshot to beutilized, this is received by the snapshot management program 12. Thesnapshot utilization command includes at least the generation number.

When the snapshot processing section 29B receives this snapshotutilization command, a snapshot of the generation number that is thusdesignated is put into a condition in which it can be utilized from themainframe (S45), by the same processing sequence as in the case of thefirst embodiment. When the snapshot has thus been put into a conditionin which it can be utilized from the mainframe, a message to that effectis communicated to the mainframe 1 (S46).

In this way, generation management can be automatically performed in thedisk array system 2.

The embodiments of the present invention described above are examplesgiven by way of explanation of the present invention and the scope ofthe present invention is not restricted solely to these embodiments. Thepresent invention could be put into practice by persons skilled in theart in various other ways without departing from the essence of thepresent invention.

For example, although, in the above embodiments, a snapshot processingsection was provided as a function of the interface control section, adisk control section could also be provided and either of the interfacecontrol section or disk control section could be implemented bydifferent processors or could be implemented by a plurality ofprocessors within the disk array system.

Also, although, in the above embodiments, the disk array system 2 holdsthe generation management information, this could be held by themainframe and generation management could be performed on the mainframe.

1. A storage system that is accessed from a host device, comprising: astorage device comprising a plurality of logical volumes; a storagedevice control section that controls said storage device; an interfacesection that receives a snapshot acquisition instruction and snapshotutilization instruction from said host device; and a snapshot processingdevice that executes processing for acquisition and utilization ofsnapshots in accordance with said snapshot acquisition instruction andsaid snapshot utilization instruction received by said interfacesection; wherein said snapshot processing device, in response to saidsnapshot acquisition instruction, selects a logical volume that iscapable of use as a snapshot target from said plurality of logicalvolumes, instructs said storage device control section to acquire asnapshot of the logical volume of the snapshot source, designated insaid snapshot acquisition instruction for the selected logical volume,and holds generation information that associates the generation numberspecified in said snapshot acquisition instruction with the address ofsaid selected logical volume, and in response to said snapshotutilization instruction, specifies a logical volume where the snapshotcorresponding to the generation number specified in said snapshotutilization instruction is held, by referring to said generationinformation, and allocates an address accessible by said host device tosaid specified logical volume.
 2. The storage system according to claim1 wherein the address accessible by said host device that is allocatedto said specified logical volume is a logical volume address that isregistered beforehand in said host device.
 3. The storage systemaccording to claim 1 wherein, when the address accessible by said hostdevice is allocated to said specified logical volume, said snapshotprocessing device adds information indicating the fact that the addressaccessible by said host device has been allocated to said specifiedlogical volume to said generation information.
 4. The storage systemaccording to claim 1 wherein the snapshot acquisition instructionfurther includes designation of the logical volume of the snapshottarget and said snapshot processing device, in response to said snapshotacquisition instruction, if the snapshot target logical volumedesignated in said snapshot acquisition instruction is already used,selects an unused logical volume from said plurality of logical volumesand gives an instruction to said storage device control section toacquire a snapshot of the snapshot source logical volume designated insaid snapshot acquisition instruction for the selected logical volume.5. The storage system according to claim 1 wherein said snapshotutilization instruction further includes designation of the logicalvolume of the snapshot target and said snap shot processing device, inresponse to said snapshot utilization instruction, if a snapshot of thegeneration number specified in said snapshot utilization instruction hasnot been acquired by the snapshot target logical volume specified insaid snapshot utilization instruction, allocates the logical volumeaddress of said specified snapshot target to the logical volume wherethe snapshot of said designated generation number is held, by referringto said generation information, and gives notification to said hostdevice of the fact that a snapshot of said specified generation numberhas become available for use.
 6. A storage system that is accessed froma host device, comprising: a storage means comprising a plurality oflogical volumes; a storage means control means that controls saidstorage means; an interface means that receives a snapshot acquisitioninstruction and snapshot utilization instruction from said host device;and snapshot processing means that executes processing for acquiring andutilizing snapshots in accordance with said snapshot acquisitioninstruction and said snapshot utilization instruction received by saidinterface means; wherein said snapshot processing means, in response tosaid snapshot acquisition instruction, if the snapshot target logicalvolume designated in said snapshot acquisition instruction is alreadyused, selects an unused logical volume from said plurality of logicalvolumes; gives an instruction to said storage device control means toacquire a snapshot of the logical volume of the snapshot sourcedesignated in said snapshot acquisition instruction for the selectedlogical volume, and holds generation information associating thegeneration number specified in said snapshot acquisition instructionwith the address of said selected logical volume; and in response tosaid snapshot utilization instruction, specifies a logical volume wherea snapshot corresponding to the generation number specified in saidsnapshot utilization instruction is held, by referring to saidgeneration information, and allocates an address accessible by said hostdevice to said specified logical volume.
 7. A method of acquiring andutilizing snapshots in a storage system that is accessed from a hostdevice, comprising a storage device comprising a plurality of logicalvolumes, the method comprising the steps of; selecting, when a snapshotacquisition instruction is received from said host device, an unusedlogical volume from said plurality of logical volumes if the snapshottarget logical volume designated in said snapshot acquisitioninstruction is already used; acquiring a snapshot of the snapshot sourcevolume designated in said snapshot acquisition instruction for theselected logical volume; holding generation information associating thegeneration number specified in said snapshot acquisition instructionwith the address of said selected logical volume; and specifying, when asnapshot utilization instruction is received from said host device, alogical volume where a snapshot corresponding to the generation numberspecified in said snapshot utilization instruction is held, by referringto said generation information; and allocating an address accessible bysaid host device to said specified logical volume.
 8. A storage systemthat is accessed from a host device, comprising: a storage devicecomprising a plurality of logical volumes; a storage device controlsection that controls said storage device; an interface section thatreceives a snapshot acquisition instruction, snapshot utilizationinstruction and generation information acquisition instruction from saidhost device; and a snapshot processing device comprising a generationinformation storage section that stores generation informationindicating the snapshot acquisition status for each generation and thatexecutes processing in accordance with said snapshot acquisitioninstruction, said snapshot utilization instruction and generationinformation acquisition instruction received by said interface section;and wherein said snapshot processing device, in response to saidsnapshot acquisition instruction, selects one logical volume from saidplurality of logical volumes; gives instruction to acquire a snapshot ofthe logical volume of the snapshot source designated in said snapshotacquisition instruction for the selected logical volume and, specifies ageneration number by referring to said generation information storagesection and associates this specified generation number with the addressof said selected logical volume and stores these in the generationinformation storage section; in response to said generation informationacquisition instruction, acquires the information stored in thegeneration information storage section and gives an instruction to saidinterface section to send the information to said host device; and, inresponse to said snapshot utilization instruction, specifies the logicalvolume where a snapshot has been acquired corresponding to thegeneration number specified in said snapshot utilization instruction byreferring to said generation information storage section and allocatesan address accessible by said host device to said specified logicalvolume.
 9. A storage system that is accessed from a host device,comprising: storage means comprising a plurality of logical volumes;storage means control means that controls said storage means; interfacemeans that receives a snapshot acquisition instruction, snapshotutilization instruction and generation information acquisitioninstruction from said host device; and snapshot processing meanscomprising generation information storage means that stores generationinformation indicating the snapshot acquisition status for eachgeneration and that executes processing in accordance with said snapshotacquisition instruction, said snapshot utilization instruction andgeneration information acquisition instruction received by saidinterface means; and wherein said snapshot processing means, in responseto said snapshot acquisition instruction, selects one logical volumefrom said plurality of logical volumes; gives an instruction to acquirea snapshot of the logical volume of the snapshot source designated insaid snapshot acquisition instruction for the selected logical volumeand, specifies a generation number by referring to said generationinformation storage means, and associates this specified generationnumber with the address of said selected logical volume and stores thesein said generation information storage means; in response to saidgeneration information acquisition instruction, acquires the informationstored in said generation information storage means and gives aninstruction to said interface means to send the information to said hostdevice; and, in response to said snapshot utilization instruction,specifies the logical volume where a snapshot has been acquiredcorresponding to the generation number specified in said snapshotutilization instruction by referring to said generation informationstorage means, and allocates an address accessible by said host deviceto said specified logical volume.
 10. A method of acquiring andutilizing snapshots in a storage system that is accessed from a hostdevice, comprising a storage device comprising a plurality of logicalvolumes and a generation information storage device that storesgeneration information indicating for each generation the snapshotacquisition status, the method comprising the steps of; selecting alogical volume from said plurality of logical volumes when a snapshotacquisition instruction is received from said host device; giving aninstruction in respect of the selected logical volume for theacquisition of a snapshot of the snapshot source logical volumedesignated in said snapshot acquisition instruction; specifying ageneration number by referring to said generation information storagesection, and associating the specified generation number with theaddress of said selected logical volume and storing these in saidgeneration information storage device; acquiring and sending to the hostdevice the information stored in said generation information storagesection when said generation information acquisition instruction isreceived from said host device; specifying a logical volume where asnapshot has been acquired corresponding to the generation numberspecified in said snapshot utilization instruction by referring to saidgeneration information storage section when said snapshot utilizationinstruction is received from said host device; and allocating an addressaccessible by said host device to said specified logical volume.